In recent years, along with increasingly strengthened international competition among different types of industries (such as competition with bio-industry), so-called cost consciousness has been raised in, among others, the field of agriculture. In the field of agriculture, similarly as in the other industries, it is extremely important to provide high-quality products as efficient as possible (in other words, at the lowest cost). However, on the basis of the particularity in products (farm products) in the agriculture and in the production processes used in agriculture (especially on the basis of the fact that such products are “living”), it is necessary to adopt specialized designs or device for use in the field of the agriculture, which is different from those in the other industries.
From ancient times, it has naturally been an essential technique in agriculture to cultivate plants (or seedlings) of good quality. However, in recent years, for various reasons, such as aging in the agricultural working population, insufficient numbers of workers in farming areas, and improvements in seedling-producing techniques, there has been a tendency for a so-called division of labor to develop wherein professional suppliers produce seedlings, and common agricultural workers purchase the seedlings as “commercially available products”.
The specialization of seedling production has necessarily provided large-scale production facilities for producing seedlings of plants. In such production facilities, of course, techniques have been energetically developed for producing a large amount of better-quality seedlings at lower cost.
In general, the growth of seedlings is greatly influenced by the environment in view of the quality and quantity thereof, and thus environment control is an extremely important factor in seedling production. Examples of the environmental conditions which need to be controlled during seedling production, i.e cultivation of plant bodies, include: lighting, temperature, humidity, amount of soil moisture and gas concentration in the soil.
Among these environmental conditions, both sunlight and artificial light have been used as a light source in traditional seedling-producing facilities. The temperature is regulated by heating or cooling the inside of a greenhouse. In addition, soil moisture is regulated by irrigation, and the gaseous environment in the soil is regulated by controlling soil gas permeability.
In the various environment control techniques described above, one particularly important factor is the regulation of soil moisture. As irrigation methods employed for the purpose of regulating soil moisture content, simple sprinkling methods (i.e. methods of sprinkling water from the upper head portions of plants by means of sprinkling devices such as watering cans and sprinklers) have been commonly used. Also employed are the drip irrigation method, the subirrigation (ebb-and-flood) method, etc. However, whichever of these methods is used, an expensive irrigation facility and correct irrigation control are required. In addition, when the most common overhead sprinkling method is used, the entirety of the plant body is wetted, including parts of the plant body near the ground and soil in the neighborhood of the earth surface, whereby the risk of damage due to disease tends to increase. In general, most plants are cultivated under a soil moisture condition corresponding to a pF value in the range of 1.5-2.5, which is a numerical value representing the soil moisture tension (With respect to the details of the “pF value”, the paper “Comprehensive Soil Theories” (DOJOU TSUURON), written by Yasuo Takai and Hiroshi Miyoshi, published by Asakura Shoten, 1977, pp. 88-89 may be referred to).
When irrigation water supplied is insufficient, growth of a plant is suppressed. On the other hand, when irrigation is excessive, the plant is softened and weakened, and rapid changes in wet and dry conditions result in water stress on the plant body, decreasing its resistance to damage from disease. As described above, it is difficult to appropriately control irrigation to a certain degree, which is greater than the difficulty which would generally be expected. In addition, excessive moisture content due to over-irrigation adversely affects the gaseous environment in the soil, resulting in possible failure of the plant to grow, and therefore it is necessary to strictly limit or control the time of irrigation, and the amount of irrigation water.
Further, with respect to the above-mentioned gaseous environment in the soil, this is characterized in that it has a lower oxygen concentration and a higher carbon dioxide concentration, as compared with those in the atmosphere. This is because oxygen is absorbed and carbon dioxide is generated by the roots and microbes present in the soil. In general, when the oxygen concentration in the soil decreases, and the carbon dioxide concentration increases relatively, plant growth is suppressed. In particular, the carbon dioxide concentration in the soil tends to become higher the greater the depth from the earth surface, whereas, conversely, the oxygen concentration tends to decrease.
The diffusion coefficient of gases in the soil is affected by the void distribution characteristic of the soil. Accordingly, when the soil moisture content is increased, voids in the soil are filled by the excessive moisture content (and the number of voids in the soil, naturally, decrease), the oxygen concentration in the soil decreases, and at the same time the carbon dioxide concentration in the soil suddenly increases, inhibiting plant growth. In particular, when voids in the soil are temporarily blocked up and the moisture content therein is stagnant at the time of irrigation, the oxygen in the soil is decreased, risking damage to the roots. It is believed that plant growth is also inhibited by high-concentration carbon dioxide, since such carbon dioxide is dissolved in the soil moisture, thereby decreasing the pH thereof. As described above, the gaseous environment condition in the soil is closely related to the soil moisture conditions.
Also as described above, in order to control the environmental conditions for facilitating good growth of plants or seedlings in conventional production facilities for producing useful plants and crops, expensive facilities and equipment are required, resulting in high running costs. Further, when conventional techniques are used, it has proven to be difficult to simultaneously achieve an optimum gaseous environment and optimum moisture environment in the soil.